The present invention generally relates to microwave dielectric loaded cavity resonators and filters, and in particular, to a tunable temperature stable dielectric loaded resonator and filter mechanism providing a wide range of resonant frequency adjustment and full range temperature stability for the dielectric loaded cavity resonators and filters.
Resonators are important components in microwave communication circuits. It is well known that dielectric loaded resonators exhibit superior performance characteristics over those of other known types of resonators. They offer high-unloaded Q in a small mechanical package. Thus, the dielectric loaded resonators are being used more frequently, particularly in narrow bandwidth, low insertion loss filters and multiplexers.
TE01xcex4 mode is usually the fundamental mode and the commonly used resonant mode for a dielectric loaded resonator. The resonant frequency of a dielectric resonator is primarily determined by the dimensions of the dielectric body when the relative dielectric constant of the material is larger than 30.
By bringing the enclosure close to the dielectric resonator, the resonant frequency of the TE01xcex4 mode is modified to a new increased value. Therefore, a typical method of changing the resonant frequency of a ceramic resonator 1 is to adjust the distance of a conductive metallic surface by a tuning plate 2 from a planar surface of the resonator housing 3, as shown in FIG. 1. However, the resonant frequency tuning range of the resonator that is changed by this method is very limited, and bringing the metal surface of the tuning mechanism close to dielectric resonator produces appreciable surface currents. As a result, the unloaded Q of the resonator is reduced.
For wider tuning range applications, a dielectric tuning plate 4 can be used to replace the metal plate 2 as shown in FIG. 2. In this case, as the dielectric tuning plate 4 is moved closer to the ceramic resonator 1, the resonant frequency decreases. The change in resonant frequency is nonlinear in relation to the change of the dielectric tuning plate 4. In addition, the resonant frequency is extremely sensitive when the dielectric tuning plate 4 is close to the main ceramic resonator body 1. Furthermore, it is very difficult to temperature compensate the resonator. A preferable way is to use a main dielectric ring resonator 5 and a smaller diameter dielectric tuning plug 6 positioned in or near the concentric main dielectric resonator hole 7, as shown in FIGS. 3 and 4. In this case, the resonant frequency change is nearly linear with respect to the dielectric plug movement.
For example, FIG. 5 shows changing frequency by movement of the ceramic plug 8. When the plug 8 is fully inserted into the resonator 9, frequency is at a minimum and with the plug 8 completely outside the resonator 9, frequency is at a maximum.
One skilled in the art will appreciate that it is usually difficult to position the dielectric body in the enclosure of the resonator. This is because the support structure must not influence the EM fields present in the resonator which can provide spurious responses. For example, FIGS. 3 and 4 show support mechanisms 11A and 11B for a resonator 5. FIG. 3 shows a lower resonator support 11A, and FIG. 4 shows a double resonator support 11B.
According to U.S. Pat. No. 5,612,655, to Stronks et al., a plastic supporting structure was used to support both the main dielectric body and the tuning element. However, this structure results in too many parts used in the assembly, and therefore the structure is very complicated. Furthermore, the structure cannot control unwanted lateral movement of the dielectric body, and the plastic material usually has a high thermal expansion coefficient, and therefore, the resonator lacks thermal and long term stability. In addition, as the thermal conductivity of the plastic is generally poor, it limits the average power handling of the resonator and filter.
High purity alumina can be used as support material to improve the thermal conductivity of the resonator from the main dielectric body to the resonator housing, because it has a low loss and a relative high thermal conductivity. However, as alumina is a very rigid ceramic material, it is very difficult to affix the dielectric body using alumina as the support in the resonator housing. As a result, such a resonator assembly is unreliable.
Furthermore, another problem with the tuning dielectric body of previous structures or methods is that they all must be assembled and installed prior to final resonator assembly. As a result, no replacement and repair of the tuning element is allowed after resonator assembly, which is not suitable for tunable resonators, filters, and mass production.
It is therefore an object of the invention to provide a dielectric resonator with a wide tuning range and temperature stable range while maintaining a high unloaded Q.
It is another object of the invention to provide a dielectric resonator that is simple and easy to manufacture in addition to being durable.
It is a further object of the invention to provide a dielectric resonator which can be repaired and tuned after resonator assembly.
According to an exemplary embodiment, a resonator assembly comprising a conductive cavity, a main dielectric body, a tuning element assembly and a plastic top support structure is provided.
A tunable, temperature compensated, thermally and mechanically stable dielectric loaded cavity resonator and filter assembly having high unloaded Q, a wide frequency tuning range and a simple structure suitable for high volume production is provided according to the present invention. The cavity resonator consists of a conductive housing, a substantially cylindrical ring-shaped dielectric body with a low loss, low thermal expansion coefficient support, a tuning mechanism and a plastic support at the opposite side of the main cylindrical dielectric body, that holds the main cylindrical dielectric body in place. The tuning mechanism further includes a substantially cylindrical dielectric tuning element positioned in or near the hole of the main cylindrical dielectric body and a self-locked or equivalent nut locked rotor with a support which is preferably made of the same material as that of the main dielectric cylindrical body support. The rotor is accessible and rotationally movable from the outside of the conductive enclosure, resulting in linear motion of the dielectric tuning element with respect to the main dielectric body. Therefore the resonant frequency of the resonator can be substantially adjusted.